Such sensor systems for determining a physical, measured variable, especially fill-level in a container, are used frequently in measuring devices of automation and process-control technology. The assignee, for instance, produces and distributes measuring devices under the name, Prosonic, FMU. These work according to the travel-time measurement method and serve for determining and/or monitoring fill-level of a fill substance in a container. In the travel-time measuring method, for example, ultrasonic sound signals produced via an ultrasonic transceiver are transmitted into the process space, and the reflected echo waves are, following a distance-dependent travel time of the signals, received back by a transmitting/receiving element. From the time difference between transmission of the ultra-/sonic signals and receipt of the reflected echo signals, separation of the measuring device from the fill-substance can be ascertained. Apparatuses and methods for determining fill-level via travel-time of ultrasonic signals, or other signals, such as e.g. radar, utilize a physical law, according to which the traveled distance is equal to the product of travel time and velocity of propagation. Taking into consideration geometry of objects installed in the container and the container itself, the fill level of the fill substance is then ascertained as a relative, or absolute, quantity.
Production of sound waves, e.g. ultrasonic waves, and the detecting of the reflected echo waves following a distance-dependent travel time can be done separately by separate transmitting and receiving elements or by combined transmitting/receiving elements. In practice, most often, only a single transmitting/receiving element—a so-called ultrasonic transceiver—is used, which produces a transmission signal and receives, displaced in time, a reflected, or echo, signal.
Travel time of ultrasonic waves depends on propagation velocity of the ultrasonic waves in the gas phase above the medium to be measured and is generally referred to as the gas-phase velocity. This gas-phase velocity is, among other things, strongly dependent on the temperature of the gas phase, in which the ultrasonic wave is propagating. For this reason, temperature of the medium must be known for determining gas-phase velocity, in order that the travel time of the ultrasonic signals can be converted to a corresponding traveled distance.
Additionally, an ultrasonic sensor is excited in such a manner that it oscillates at the resonance frequency. In this resonance-frequency mode of the ultrasonic sensor, a highest possible transmitted power of the ultrasonic signal is achieved. The resonance frequency of the ultrasonic sensor depends both on the geometric variables of the sensor and its sensor element, as well as also on surrounding temperature.
DE 103 23 063, DE 42 33 257 C1 and DE 42 23 346 C2 describe fill-level measuring devices with ultrasonic sensors, wherein the resonance frequency of the ultrasonic sensors is determined at different temperatures. Integrated into these ultrasonic sensors are temperature sensors serving for registering the current, surrounding temperature of the ultrasonic sensor. On the basis of the current, surrounding temperature, the resonance frequency of the ultrasonic sensors is tracked and matched by the measurement transmitter by adjusting the excitation during transmission operation. From current values and historical values of the resonance frequency and the surrounding temperature, also a statement can be made concerning deposition of medium on the ultrasonic sensor, i.e. concerning the matter of so-called accretion.
DE 198 08 994 C2 describes, in contrast, an apparatus, which, by means of a temperature compensation capacitor in the ultrasonic sensor, counteracts a deviation of the temperature-dependent resonance frequency of the ultrasonic transducer brought about by a change of the surrounding temperature.
In process plants, there are various measuring conditions, such as, for example, container geometries, range, measured-value resolution, measured medium, which can, most often, not be covered alone by a single ultrasonic sensor. In order that these different ultrasonic sensors can operate at their respective resonance frequencies, the measurement transmitter must drive them with appropriate exciting signals. For this, it is necessary to know the type of attached ultrasonic sensor. One possibility is to allow the type of connected ultrasonic sensor to be automatically ascertained by an integrated sensor identifier, for example in the form of an identifying resistance. Such an identifying resistance is integrated in DE 40 35 403 A1 in the sensor body. The identifying resistance is read by the control unit, or the measurement transmitter, in that a fixed connection is provided between sensor and control unit, and the type of sensor is then established.
For independent evaluation of the sensor identifier and the temperature-measuring element by the measurement transmitter, in usual devices, the measurement transmitter evaluates both separately from one another. This causes an increased effort as regards connecting and measuring between sensor and measurement transmitter.